Electroplating a gold-platinum alloy and electrolyte therefor

ABSTRACT

An electroplating bath for the deposition of gold-platinum alloys comprising an aqueous solution of alkali metal hexahydroxyplatinate and alkali metal aurate. Preferably, the solution comprises sufficient alkali metal hexahydroxyplatinate to provide from substantially 15 to substantially 25 grams of platinum per liter and sufficient alkali metal aurate to provide from substantially 0.5 to substantially 3.0 grams of gold per liter. The bath is operated at a temperature preferably in the range of from substantially 50*C to substantially 65*C.

United States Patent 1191 1111 B 3,923,612 Wiesner Dec. 2, 1975 54 lELECTROPLATING A GOLD PLATINUM OTHER PUBLICATIONS ALLOY AND ELECTROLYTETHEREFOR [75] Inventor: Harold J. Wiesner, Livermore, Graham et Plating148453 (1949). Calif.

[73] Assignee: The United States of America as Primary Examiner cemld LKaplan represented by the Umted states Attorney, Agent, or Firm-John A.Horan; Frederick Energy Research and Development A. Robartson; IreneCroft Administration, Washington, DC.

[22] Filed: Feb. 25, I974 [57] ABSTRACT [2] 1 Appl' 445740 Anelectroplating bath for the deposition of gold- [44] Published under theTrial Voluntary Protest platinum alloys comprising an aqueous solutionof al- Program on January 28, 1975 as document no. kali metalhexahydroxyplatinate and alkali metal au- B 445,740. rate. Preferably,the solution comprises sufficient alkali metal hexahydroxyplatinate toprovide from sub- [52] US. Cl. 204/43 G; 75/165; 75/172; stantially 15to substantially 25 grams of platinum per 204/43 N liter and sufficientalkali metal aurate to provide from [51] Int. CI. C25D 3/62; C25D 3/56substantially 0.5 to substantially 3.0 grams of gold per [58'] Field ofSearch 204/43 G, 43 N, 44, I23 liter. The bath is operated at atemperature preferably in the range of from substantially 50C tosubstantially [5 6] References Cited 65C.

FOREIGN PATENTS OR APPLICATIONS 6/1963 United Kingdom 204/43 G 6 Claims,N0 Drawings ELECTROPLATING A GOLD-PLATINUM ALLOY AND ELECTROLYTETHEREFOR BACKGROUND OF THE INVENTIQN The invention described herein wasmade in the course of, or under, Contract No; W-7405-ENG-48 with theUnited States Atomic Energy Commission.

This invention relates to the electrodeposition of gold-platinum alloys.I

For the last two decades great impetus to gold plating research has beengiven ,by the requirements of the electronics and allied industries thatspecified thicker coatings of greater hardness, wearresistance,corrosion resistance and lower porosity; Another field which is openingfor electrodeposited gold alloys is that of high temperatureapplications. Electroplated gold-copper alloys have been investigatedfor this purpose --but, in general, a major disadvantage. ofthesealloysis their poormechanical properties at high temperature. Also, thelow-density copper component undesirably decreases the density of thealloy. Of the binaryalloysinvestigated for high temperatureuse,gold-platinum alloys have been found to be the most suitable. Platinumhardens gold very effectivelyand also increases its resistance tooxidation and-corrosion. Gold-platinum alloys have been used for manyyearsas spinning jets in the viscose rayon industry where theywithstandcorrosive conditions and severe mechanical conditions for long periods.The gold-platinum alloys arealso suitable for use as crucibles foralkaline fusions.

Electrodeposition of alloys involves; more complicated processes thanthe deposition of ,pure metals. Alloy deposition is much more sensitiveto a variety of factors than a'single metal. Variables-must becontrolled much more closelythan in a single metal deposition ino'rderto obtain deposits of uniform color, corrosion resistance,composition and appearance. The supporting electrolyte must be chosen sothat the reduction potential of the metals will, be fairly close.Indeed, it is often difficult to predict the composition of an alloyfrom thatof the electrolyte andfrom the plating conditions. I

to insure deposition of platinum on the cathode. It has been found thata'concentration of alkali metal hexahydroxyplatinate sufficient toprovide from substantially to substantially grams of platinum'per literof aqueous solution is eminently suitablefor use in the presentinvention. For best results, an amount of plati-Electrodepositionprocedures for such gold alloys as v gold-copper andgoldsilver alloys, generally employ cyanide complex solutions. However,metals of the platinum group are not deposited from aqueous cyanidesolutions since the cyanide complexes of these metals are so stablethattheir deposition on the cathodeis either impossible, or possibleonly at very low eurrent densities. Investigations of thedepositionofgold-platinum alloys from halide systems were made andreported by A. K. Graham, S. Heiman, and H. L. Pinkerton, flaring,

SUMMARY-OF THE INVENTION.

It has been found that gold-platinum alloy deposits of consistentlyexcellent quality canbe obtained by electrodeposition from an aqueouselectroplating bath comprising an alkali metal hexahydroxyplatinate andan alkali metal aurate. The concentration of the alkali metalhexahydroxyplatinateshould be sufficiently high numin solution withinthe range of from substantially 19 to substantially 22 grams per literis preferred, with the concentration of the alkali metal aurate beingadjusted in accordance with the concentration of gold desired in thefinaldeposit. Deposits containing from substan'tially 15% tosubstantially 95% gold can be obtained with the baths of the presentinvention. For high temperature applications, alloy deposits containingfrom substantially=% to substantially gold are generally preferred. Oneof thevariables requiring control in the utilizationof the baths of thepresent invention is the pH of the solution. It is important that theelectroplating bath be'highly alkalineythe pH of the solution should bemaintained above substantially 11.0. If the pH of the solution-fallsbelow substantially llIO, the alkali metal hexahydroxyplatinate tends toform a precipitate. Generally, the solutions preparedby dissolving analkali metal hexahydroxyplatinate and an alkali metal aurate have asufficiently high pH for use in the present invention, but, ifnecessary, the pH of the solution can be adjusted before and/or duringuse by the addition of suitable amounts of water-soluble alkaline actingsubstances such as alkali metal'salts of weak acids, e.g., phosphoricacid, acetic acid or citric acid.

- Another variable requiring control is the plating temperature'Thetemperature affects not only the structure and appearance-oftheelectrodeposited alloys but also theirchemic'al"composition.'Ingeneral, the preferred temperature is in the range of fromsubstantially 50C to substantially 65C.

DETAILED DESCRIPTION OF THE INVENTION According to the presentinvention, there is provided an electroplating bath for theelectrodeposition of goldplatinum alloys comprising an alkalimetal-hexahydroxyplatinate and tion." 1

In a specific embodiment of the invention, the aqueous electroplatingbath comprises from substantially 15 and from substantially 0.5 tosubstantially 3.0, preferably from substantially 1.0 to substantially2.0, grams per liter of gold as sodium or potassiumaurate, the pH of thesolution being maintained at above substantially l 1.0, preferably abovesubstantially ll.5.

The operatingconditions suitable for any particular plating -applicationmay be readily' determined by one skilled in the art. The averagecathode current density used will depend upon the alloy desired and onthe shape and size of the article to be plated. It is preferable analkali metal aurate i'n aqueous soluto employ cathode current densitiesof below substantially 20 amperes/ft? preferably substantially 5 tosubstantially 8 amperes/ft under normal direct current platingconditions, with a deposition rate of from substantially 0.0002 tosubstantially 0.00025 inch/hr.

Generally, the electrolyte solution is used with insoluble platinum orplatinum clad anodes, and it is necessary to replenish the gold andplatinum deposited from the electrolyte solution by addition of freshgold and platinum compounds.

As stated above, it is important that the pH of the solution bemaintained above substantially l 1.0, and preferably above substantiallyl 1.5, to prevent precipitation of the alkali metalhexahydroxyplatinate. lf necessary, water-soluble alkaline substancessuch as alkali metal salts of weak acids, e.g., potassium phosphate,potassium acetate, or potassium citrate can be added before and/orduring use in sufficient quantities to maintain the pH of the solutionin the required range. Suitable brightening agents, which are well knownin the art, can be added from time to time to maintain the desiredphysical characteristics of the alloy deposited.

For best results, the electrolyte of the present invent-ion is operatedat temperature of substantially 50C to substantially 65C. I

The following examples are illustrative of the electroplating solutionsof the present invention.

, EXAMPLE I A 250 m1. aqueous electroplating solution was preparedcontaining 21 grams per liter of platinum as K Pt( OH) and 0.7 grams perliter of gold as KAuO The .pH .of the solution was above 11.5. The bathwas operated at a temperature of 55C. The electrolyte was used to plateclean brass parts. The composition of the deposited alloy was 10% Au-90%Pt. The deposit was found to be smooth.

. EXAM PLE [I An aqueous electroplating solution was prepared as inExample 1, except that theconcentration of gold present (as KAuO was 2.8grams per liter. The electrolyte was used to plate clean brass parts.The bath was operated at a temperature of 65C. A current of 0.2 amperewas employed. The composition of the final deposit was 30% Pt-70% Au.The deposit was found to be of excellent quality.

EXAMPLE III A 250 ml. aqueous electroplating solution was preparedcontaining grams per liter of platinum as K Pt(OH and 1.55 gram perliter of gold as KAuO The pH of the solution was above 1 1.5. The bathwas operated at a temperature of 53C and 0.4 ampere was used to plate aclean brass panel. The final deposit contained 37.538% Pt.

EXAMPLE [V A bath containing 2.0 g/lAu (as KAuO and about 20 g/l Pt as KPt(OH) operated at 5355C and a current of 0.2 amperes produced a depositcontaining 55% Pt.

The electroplating solutions illustrated in the above examples can alsobe prepared with other alkali metal hexahydroxyplatinates, particularlyNa Pt(OH) and 4 with other alkali metal aurates, particularly NaAuOHowever, from a long range stability standpoint, the potassium salts arepreferred.

Alloy deposits containing from substantially 15% to substantially goldcan be obtained by using electroplating baths similar to thoseillustrated above. For best results, the concentration of the alkalimetal hexahydroxyplatinate should be such as to provide fromsubstantially 15 to substantially 25 grams, and preferably fromsubstantially 19 to substantially 22 grams, of platinum per liter ofsolution, with the concentration of the alkali metal aurate beingadjusted to provide the desired concentration of gold in the finaldeposit. Generally, varying the concentration of the alkali metal aurateto provide from substantially 0.5 to substantially 3.0 grams of gold perliter of solution will yield goldplatinum alloy deposits within thecomposition range contemplated by the present invention providing aconstant current density is used. Thus, to maintain a specific alloycomposition, the platinum and gold contents in the bath must becarefully controlled as well as the current density.

Although the invention has been described with reference to specificexamples, it will be appreciated that various modifications and changeswill be evident to those skilled in the art without departing from' thetrue spirit and scope thereof. Thus, it is not intended to limit theinvention except by the terms of the following claims.

What 1 claim is:

1. An aqueous electrolyte solution for the electrodeposition ofgold-platinum alloys comprising an alkali metal hexahydroxyplatinate andan alkali metal aurate and wherein the pH of the solution is abovesubstantially ll.0.

2. An electrolyte solution as claimed in claim 1 wherein the alkalimetal hexahydroxyplatinate is selected from the group consisting ofsodium hexahydroxyplatinate and potassium hexahydroxyplatinate, and thealkali metal aurate is selected from the group consisting of sodiumaurate and potassium aurate.

3. An electrolyte solution as claimed in claim 1 wherein theconcentration of the alkali metal hexahydroxyplatinate is sufficient toprovide from substantially 15 to substantially 25 grams of platinum perliter of solution.

4. An electrolyte solution as claimed in claim 1 wherein theconcentration of the alkali metal aurate is sufficient to provide fromsubstantially 0.5 to substantially 3.0 grams of gold per liter ofsolution.

5. An electrolyte solution as claimed in claim 1 wherein the alkalimetal hexahydroxyplatinate is potassium hexahydroxyplatinate in aconcentration sufficient to provide from substantially 19 tosubstantially 22 grams of platinum per liter of solution and the alkalimetal aurate is potassium aurate in a concentration sufficient toprovide from substantially 0.5 to substantially 3.0 grams of gold perliter of solution.

6. A process for the electrodeposition of goldplatinum alloys whichcomprises electrolyzing an aqueous electrolyte solution as claimed inclaim 1 at a.temperature between substantially 50C and substantially65C.

1. AN AQUEOUS ELECTROLYTE SOLUTION FOR THE ELECTRODEPOSITION OFGOLD-PLATINUM ALLOYS COMPRISING AN ALKALI METAL HEXAHYDROXYPLATINATE ANDAN ALKALI METAL AURATE AND WHEREIN THE PH OF THE SOLUTION IS ABOVESUBSTANTIALLY 11.0.
 2. An electrolyte solution as claimed in claim 1wherein the alkali metal hexahydroxyplatinate is selected from the groupconsisting of sodium hexahydroxyplatinate and potassiumhexahydroxyplatinate, and the alkali metal aurate is selected from thegroup consisting of sodium aurate and potassium aurate.
 3. Anelectrolyte solution as claimed in claim 1 wherein the concentration ofthe alkali metal hexahydroxyplatinate is sufficient to provide fromsubstantially 15 to substantially 25 grams of platinum per liter ofsolution.
 4. An electrolyte solution as claimed in claim 1 wherein theconcentration of the alkali metal aurate is sufficient to provide fromsubstantially 0.5 to substantially 3.0 grams of gold per liter ofsolution.
 5. An electrolyte solution as claimed in claim 1 wherein thealkali metal hexahydroxyplatinate is potassium hexahydroxyplatinate in aconcentration sufficient to provide from substantially 19 tosubstantially 22 grams of platinum per liter of solution and the alkalimetal aurate is potassium aurate in a concentration sufficient toprovide from substantially 0.5 to substantially 3.0 grams of gold perliter of solution.
 6. A process for the electrodeposition ofgold-platinum alloys which comprises electrolyzing an aqueouselectrolyte solution as claimed in claim 1 at a temperature betweensubstantially 50*C and substantially 65*C.